Mac II and Mac IIxLow End Mac Reader Specials
The Mac II was Apple's first modular Macintosh. Using a 68020 CPU instead of the older 68000, it could address far more memory using a 32-bit addressing mode. Although it took System 7 to provide the 32-bit option and Mode32 to let the II run in 32-bit mode, this allowed users to have more than 8 MB in their systems. However, there were other limitations: without a PMMU or 68030 upgrade, the Mac II couldn't use 4 MB SIMMs, and without the SuperDrive upgrade, high density SIMMs couldn't be used in Bank A. The IIx is essentially a Mac II motherboard revised to use a 68030 CPU. As a newer design, the IIx did not require any special upgrades before high density RAM could be installed. The greatest drawback in upgrading RAM in the II and IIx is the location of the SIMM sockets. You need to remove the entire drive support assembly (holds floppy and hard drives) to access the memory sockets. When upgrading, be careful not to damage the SIMM sockets, especially if you are removing memory. Although both machines shipped with 1 MB of memory, odds are pretty good they've already been upgraded beyond that point. Looking at the motherboard with the connectors to the rear and power supply to the right, you will see eight SIMM sockets. The four nearest the right side of the board are Bank A. The other four are Bank B. Memory must be installed in sets of four 30-pin SIMMs rated at 120ns or faster. A 1 MB configuration has four 256 KB SIMMs in Bank A; a 2 MB configuration has 256 KB SIMMs in all eight sockets. A 4 MB setup has four 1 MB SIMMs in Bank A; Bank B is empty. For 5 MB, Bank A has 1 MB SIMMs and Bank B had 256 KB SIMMs. The 8 MB configuration, which seems quite common, has 1 MB SIMMs in all 8 sockets. This is the most memory a Mac II can work with unless you have a PMMU or a 68030 accelerator. Any configuration beyond 8 MB requires some version of System 7 and Mode32. You muse select 32-bit addressing in the memory control panel to address more than 8 MB of RAM. Finally, 4 MB and 16 MB SIMMs must be PAL SIMMs; conventional 30-pin SIMMs will not work in these computers. Remember, a Mac II must have either a PMMU or 68030 accelerator and the SuperDrive upgrade before you can use 4 MB or 16 MB SIMMs in Bank A.
For 16 MB, 17 MB, or 20 MB, you must place four 4 MB SIMMs in Bank A. Leave Bank B empty for 16 MB, populate it with 256 KB SIMMs for 17 MB, or with 1 MB SIMMs for 20 MB. To reach 32 MB, insert a 4 MB SIMM in each SIMM socket.
After installing memory and reconnecting your drives, boot your Mac with extensions off (hold down the shift key) and check "About This Macintosh" under the Apple menu. If it doesn't give the expected number, you should reseat your memory. Once you know the upgrade is a success, bolt everything back in place, attach the cover, and enjoy the extra memory. I suggest you increase the size of the disk cache for better performance.
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